The Origin of the Domestic Dog, Canis familiaris By: Jessie Zgurski
The earth trembled and a great rift appeared, separating the first
man and woman from the rest of the animal kingdom. As the
chasm grew deeper and wider, all other creatures, afraid for
their lives, returned to the forest - except for the dog, who after
much consideration, leapt the perilous rift to stay with the humans
on the other side. His love for humanity was greater than
his bond for other creatures, he explained, and he willingly
forfeited his place in paradise to prove it.
-Native American folk tale, as quoted in The Lost History of the Canine Race,
by Mary Elizabeth Thurston.
Introduction
The relationship between dogs and humans dates back at least 14 000 years, and during this time, the dog has evolved to become one of the most variable animal species. Today, anywhere from 300 - 400 distinct dog breeds exist, which vary dramatically in size from three to over 150 pounds and which display an astounding amount of variation in coat type, coat colour, and general morphology. The behaviours dogs display can often vary substantially from breed to breed as well and because of this, dogs are used by humans today to perform a large variety of tasks, including herding or guarding livestock, pulling sleds, tracking or retrieving game animals, destroying vermin, locating lost people or disaster victims, and assisting handicapped people. However, as early as 14 000 years ago, there were no domestic dogs. There were wolves, coyotes, and jackals - all potential ancestors of the dog - but no dogs. The questions as to where the dog originated, the identity of the wild canine that gave rise to the dog, and, most interestingly, why and how it became domesticated and associated with humans are all rather controversial topics. This essay will focus on several aspects of the evolution of the domestic dog, and will review recent studies that have focussed on identifying the dog's closest ancestor, determining the age of the dog and discovering its location of origin. The question as to whether the dog is a result of natural or artificial selection will also be addressed.
The Dog's Ancestor
The domestic dog is a member of the family Canidae, a diverse group of carnivores which contains 36 extant species. These include wolves, jackals, and foxes, as well as the coyote, the maned wolf, the bush dog, the African wild dog, the Dhole and the racoon dog. Within this family, the dog is most closely related to wolves, jackals and the coyote, as these canines all have the same number of chromosomes and are all capable of interbreeding to produce fertile offspring. This makes all of these species potential dog ancestors, and all have been suggested at one point to have played a part in the dog's ancestry. In fact, because of the extreme variation that exists between different dog breeds, it has been assumed in the past that multiple species of canids have shared in the ancestry of the domestic dog. For example, the Nobel prize winning ethologist Konrad Lorenz once theorized that both wolves and jackals were the ancestors of the modern dog, with some breeds being the descendants of wolves, and others being the descendants of jackals (Lorenz, 1954). However, many morphological and behavioural studies have suggested that dogs are likely the descendants of wolves (see Zimen, 1981).
Recently, the study of DNA sequences has been used to address the question of the dog's ancestry, and studies using this approach have all pointed to the wolf as being the dog's closest wild relative. For example, a study done by Vila et al. (1997) examined mitochondrial DNA (mtDNA) sequences from several canines to investigate the origins of the domestic dog. mtDNA sequences were obtained from 162 wolves representing 27 populations from Europe, the Middle East, North America and Asia, 140 dogs representing 67 breeds, and 5 coyotes, 2 golden jackals, 2 black-backed jackals and 8 Abyssinian jackals. mtDNA is inherited only maternally, unlike nuclear DNA (which is found in the nucleus of the cell), which is inherited from both the mother and the father. The region of mtDNA sequenced (the control region) region was chosen because it evolves very rapidly, and since dogs are such a young species, only a rapidly evolving region of DNA would show any differences between different dog types.
The study found that dog sequences varied from jackal or coyote sequences in at least 20 places and that wolf and dog sequences were very similar - identical in one case. This suggests that the wolf is the ancestor of the dog. A analysis of the dog and wolf sequences showed that dogs may have come from at least two unique common ancestors, which suggests that the domestication of the dog may have occurred at least twice. It is also interesting to note that individuals of the same breed sometimes had very different sequences, so it seems that there was a great deal of hybridization between various dog types before modern pure breeds were established. The research team could not figure out which groups of modern wolves are the ancestors of today's wolves and noted that this ancestral wolf population may now be extinct. Other studies on domestic dog evolution which also used sequences of mitochondrial DNA to address the problem also concluded that the wolf is the ancestor of the dog and that the dog was domesticated more than once (see Tsuda et al., 1997).
Today there is little doubt that the dog did evolve from the wolf, although it may be possible that occasional interbreeding between dogs and jackals or coyotes has occurred throughout time. Dogs and wolves are considered by some biologists to be the same species, with the dog (Canis lupus familiaris) being a domestic variant or subspecies of the wolf (Canis lupus). However, Canis familiaris is still used by most biologists as the scientific name for the dog.